Polymer materials are classically subdivided in two main classes, thermosets and thermoplastics, according to their thermal behavior. The first synthetic thermosets for practical applications were invented in 1907 by Leo Baekeland by the combination of formaldehyde and phenol. The resulting highly cross-linked network gave a rigid material that does not soften when heated, and due to its versatility it was also called “the material of a thousand uses” (D. Crespy, M. Bozonnet, M. Meier, Angew. Chem. Int. Ed. 2008, 47, 3322). Today, thermosets are used for many demanding applications because of their dimensional stability, creep resistance, chemical resistance and stiffness. Because of their molecular architecture, thermosets cannot be reshaped, processed or recycled after full curing. In contrast, thermoplastics can flow upon heating enabling multiple and easy processing, such as extrusion (N. G. Gaylord, J. R. Van Wazer, Journal of Polymer Science 1961, 51, S77), as well as recycling in many cases (M. Biron, Thermoplastics and Thermoplastic Composites: Technical Information for Plastics Users, Elsevier Science, 2007). Designing polymer systems which would combine the benefits of traditional thermosets with the ‘plastic’ properties that facilitate processing is therefore a challenge that has recently attracted great interest.
A way to make the combination of these properties is offered by the introduction of exchangeable chemical bonds into a polymer network, leading to dynamic cross-links. These bonds should be able to rearrange themselves in a reversible manner, providing on a molecular level a mechanism for macroscopic flow without risking structural damage. Polymer networks containing such exchangeable bonds, also known as covalent adaptable networks or CAN's (a) C. J. Kloxin et al., Macromolecules 2010, 43, 2643; b) C. N. Bowman, C. J. Kloxin, Angew. Chem. Int. Ed. 2012, 51, 4272; c) C. J. Kloxin, C. N. Bowman, Chem. Soc. Rev. 2013, 42), may be further classified into two groups: those relying on dissociative exchange reactions and those relying on associative exchange reactions (a) T. F. Scott, et al., Science 2005, 308, 1615; b) R. Nicolaÿ et al., Macromolecules 2010, 43, 4355; c) Y. Amamoto, et al., Angew. Chem. Int. Ed. 2011, 50, 1660).
The most common ‘dissociative’ group of CAN's relies on a simple reversible covalent bond formation between two groups attached to the polymer chains. By triggering the reversed bond forming step (bond dissociation) the material can achieve topology rearrangements (stress relaxation and flow) simply because of a decrease in connectivity during the temporary depolymerization, resulting in a strong and sudden viscosity drop. Such systems will always present a sol/gel transition and can thus be solubilized in the presence of solvent. A well-known example of a thermally triggered dissociative CAN relies on the well-known reversible Diels-Alder reaction between furans and maleimides (X. Chen, Science 2002, 295, 1698).
A less common type of CAN's makes use of associative bond exchanges between polymer chains, in which the cross-links between polymer chains are only broken once another bond to another (part of the) polymer chain has been formed. As a result, such systems can change their topology with no loss of connectivity during the dynamic reorganization process, making such networks effectively permanent and insoluble even at (very) high temperatures. Interestingly, as with all chemical reactions, the rate of this associative exchange increases with the temperature, leading to an Arrhenius-like viscosity dependence rather than a sudden and marked viscosity drop at the sol/gel transition. Thermally triggered associative CANs have been coined vitrimers, (M. Capelot et al., J. Am. Chem. Soc. 2012, 134, 7664.) because of their unique combination of insolubility and gradual thermal viscosity behavior which makes these permanent polymer networks processable just like glass.
In 2011, Leibler and co-workers introduced and demonstrated the unique properties of vitrimer materials using simple transesterification chemistry in an epoxy-based resin, cross-linked with polycarboxylic acids. Addition of a mild Lewis acid catalyst like zinc acetate to these classical resins resulted in an insoluble material which combined great mechanical properties, like classical hard epoxy resins, with the ability to be reshaped and reprocessed after full curing (D. Montarnal et al., Science 2011, 334, 965; b) M. Capelot et al., ACS Macro Letters 2012, 1, 789).
Since then, several other systems have been explored as possible vitrimer materials. Altuna and co-workers were able to produce citric acid-based polyester networks capable of some stress-relaxation even in the absence of a catalyst (internally catalyzed by unreacted carboxylic acids) (F. I. Altuna, V. Pettarin, R. Williams, Green Chemistry 2013).
Other polyester network and catalyst combinations have also been explored. (J. P. Brutman et al., ACS Macro Letters 2014, 607). Vitrimer(-like) materials have also been reported based on olefin or disulphide methathesis reactions (Y.-X. Lu, Z. Guan, J. Am. Chem. Soc. 2012, 134, 14226; Y.-X. Lu, et al. J. Am. Chem. Soc. 2012, 134, 8424; A. Rekondo, et al., Materials Horizons 2013; Z. Q. Lei, et al., Chem. Mater. 2014, 26, 2038).
Although different chemistries for vitrimer materials have been explored, the reported materials all have relatively low Tg's (<57° C.) and their mechanical properties are often not comparable to those of commercial resins.
There is therefore a need to develop materials having the property of being able to be heated to temperatures such that they become fashionable without suffering destruction or degradation of their structure.